A rapid and accurate determination of the position of a person carrying a wireless mobile communication device is often useful. For example, emergency services personnel can operate more effectively and more safely if their position can be accurately determined in hazardous environments, such as within a burning building. Various prior art methods thus exist for determining the position of mobile nodes in wireless communication networks.
According to one prior art method, round trip timing is used to calculate distance. A master or ranger radio transmits a signal, such as an RTS (ready to send) signal, which is received by a target or pinger radio. The pinger radio transmits a response signal, such as a CTS (clear to send) signal, at some time after receiving the RTS signal. The CTS signal is then received at the ranger radio some time later. The time between transmitting the RTS signal and receiving the CTS signal is measured by the ranger radio, using an internal clock, as a round trip time TRTT. The time between receipt of the RTS signal and transmission of the CTS signal is measured by the pinger radio, using an internal clock, as a turn around time, TTAT. The time of flight, TTOF, between the ranger radio and the pinger radio then can be calculated by subtracting TTAT from TRTT and dividing by two.
Geographical positioning of a mobile node can be achieved by having the mobile node perform the above time-of-flight ranging transaction with three or more geographically fixed nodes (or at least nodes which can report their geographic location at any given time). The location of the mobile node is then calculated using the fact that electromagnetic wave transmissions travel at the speed of light, thus allowing range determinations, and further using known triangulation techniques.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.